09/12/96 16:09:35 MDT
If you build and fly model sailplanes you are probably a victim of the "better foil" conundrum. That is; your best efforts to try as many airfoils as possible are "foiled" by either the time or expense problem.The proper method for trying or evaluating several different airfoils is to install them on the same plane with the same planform. Most planes are offered with, at best, two different foil options. Beyond this you must make your own.
Most modern slope and thermal wings are bagged fiberglass/carbon over a foam core. Some are wood over a foam core. The assembly process of these wings is actually very simple and quick; especially if you have someone walk you through the first time. The supplies needed to make and bag foam core wings from scratch can be used for any wing on any plane; EXCEPT FOR THE TEMPLATES USED TO CUT THE FOAM CORES!!
Many many foam cutting machines lay dormant across the country due to the fact that cutting, sanding and polishing accurate templates is a daunting task at best. Especially when you consider that the templates may be used for two sets of cores at the most. Most experienced builders can assemble, bag and finish out a set of wings in a 48 hour period. (Less than 1/2 that time if you are sheeting with wood and glue!) The problem is that you can add an additional 3- 4 hours just to cut the templates for the cores. What if you only use them once?? I hate cutting templates!!!!
The following is the first of two methods I have come up with for cutting templates. This first method is designed for a maximum of several wings. It is a proven method that I now use at least once a week. All my planes now have at least two wings with different foils to choose from.
Take your airfoil plot right from your printer ("Compufoil") and use 3M spray adhesive to attach it to a piece of art tag or poster board. The stuff I use is from the local grocery store and is very thin and dense, although flexible. (very important qualities) Its actual thickness is .014 inches. Make more prints than you need. Several will be lost to practice and set-up.
Now, get a pair of NEW, yes NEW (I said new) fabric grade scissors. I use a NEW pair of Fiskers; modern scissors make all the difference in the world. Very carefully and accurately cut the templates from TE to LE so that any mistakes will be such that the wire passes over rather than getting caught. Accurate scissor use is a worthy skill to master. Practice practice practice!! Only use the deep section of the scissors; never cut out to the tip. Never allow them to slip backwards while opening them. Hold them securely (not too tight) against the exact spot where the last cut stopped. You have to use whichever method you feel works best for the design of the approach and run off ramps on the templates. Experience helps.
Turn on your bow/hotwire and turn up the juice. Place a piece of scrap poster board on the wire and see if it burns or changes color at all. Turn the power down so that it is significantly lower than the burn or charring temp of the poster board. Make sure to leave the poster board on the hot wire for a good 20-30 seconds for this trial. I have a digital ammeter hooked up and I set it to .5 amps less than the point where I can see the poster board barely begin to brown. At this temperature the cut will be slow for blue foam, but it will work great once you get the machine set up properly. Note that the 3M adhesive will "cut" with the hot wire so you don't have to worry about snags.
It is your choice how you hold the templates to the cores. I personally have a production table that clamps them upright. You can spray glue them to the edge of the foam or pin them as the instructions say in the "Feather Cut" manual. You will need to alter the weight on your bow and on the pull strings to work with the templates. Once you get it working you will not need to do the R&D again, just remember your settings. Note that my table holds these paper templates upright with no side support. They have more than enough strength on their own to support a properly set up/weighted foam cutter bow/system. I do have to help the bow up the approach ramp, but I have to do that on some of my "hard" templates too.
The second method I have come up with is the same as the first. The difference is that you spray glue the templates to a VERY thin piece of aluminum/steel shim stock instead of the posterboard. The metal is so thin that you can cut it with the same scissors I mentioned earlier. The templates are much more permanent and reusable. The shim stock can be found at a sheet metal supply house. Take your scissors to test, it should cut very easily.
Joe Galletti
The Torque & Recoil Club
7004 Chinook
Austin, TX 78736
512-454-0061
fax 512-301-1782